(CNN) -- Three Democratic senators on Monday filed a lawsuit challenging the appointment of acting Attorney General Matt Whitaker, ratcheting up the court effort to declare his placement atop the Justice Department as unconstitutional.

Sens. Richard Blumenthal of Connecticut, Sheldon Whitehouse of Rhode Island and Mazie Hirono of Hawaii filed the suit in US District Court on Monday, represented by the progressive public interest groups Protect Democracy and the Constitutional Accountability Center.